Method of making e m f cell

ABSTRACT

A heat-activated electromotive force (e.m.f.) cell having an anode formed of aluminum and a cathode comprising an oxidizing material such as sulfur. The cathode material is supported in a container or in a matrix such as porous carbon. An aluminum salt layer electrolyte such as AlCl separates the anode from the cathode. To minimize vaporization of the aluminum salt, an alkali halide salt may be mixed therewith. The cell may be operated at temperatures up to a point where either the cathode material or the electrolyte is molten.

United States Patent 1151 3,635,765

Greenberg [4 1 Jan. 18, 1972 [54] METHOD OF MAKING E M F CELL 3,238,4373/1966 Foster et al. ..l36/83 3,463,670 8/1969 Rao et al 1 36/83 [72]lnventor: Jacob Greenberg, Pepper Pike, Ohio [73] Assignee: The UnitedStates of America as Primary Examine' {\mhony skapars represented by theAdminlstrawr of the Attorney-N. T. Mus1al,G. E. Shook and G. T. McCoyNational Aeronautics and Space Adminis- V tratlon [22] Filed: June 5,1970 [57] ABSTRACT [21] Appl' No" 57,399 A heat-activated electromotiveforce (e.m.f.) cell having an R l t d [1,3, A li ti D t anode formed ofaluminum and a cathode comprising an oxidizing material such as sulfur.The cathode material is sup- [62] Dmslon of 78791 1968 ported in acontainer or in a matrix such as porous carbon. An aluminum salt layerelectrolyte such as AlCl separates the [52] US. Cl ..l36/83 R, 136/100R, 136/175 anode from the cathode To minimize vaporization of the [5111131. CL minum salt an alkali halide salt y be mixed therewith The [58]Fleld of Search ..l36/83,90, 6,100, 175-176, cell may be operated attemperatures up to a point where 1 137 either the cathode material orthe electrolyte is molten. [56] References Cited 2 Claims, 4 DrawingFigures UNITED STATES PATENTS 2,713,539 9/1955 Bradshaw et 211... ..13s3 PATENIED JAN 1 8 I972 FIGJ FIG. 3

FIG. 4

mvemoa I JACOB GREENBERG ATTORNEYS METHOD OFMAKING E M F CELL STATEMENTOF COPENDENCY This is a division of application Ser. No. 787,911 filedDec.

30, 1968, now US. Pat. No. 3,573,986.

ORIGIN OF THE INVENTION This invention described herein was made by anemployee of the UnitedStates Government and may be manufactured and usedby or for the Government of the United States of America forgovernmental purposes without the payment of any royalties thereon ortherefor.

BACKGROUND OF THE INVENTION This invention relates to heat-activatede.m.f. cells and is directed more particularly to a heat-activatede.m.f. cell having an aluminum anode.

Because of its low-equivalent weight,relatively high-energy density,ease of handling, and availability in many inexpensive forms, aluminumis a very desirable material for use as battery anodes. In the past,some e.m.f. cells have been constructed using aluminum anodes withaqueous electrolytes. Such e.m.f. cells have not been successful becauseof corrosion which occurs on the aluminum anode. This corrosion causesunacceptably low efiiciency in aluminum anode cells.

Accordingly, it is an object of the invention to provide an improvedaluminum anode cell.

It is another object of the invention to provide an aluminum anode cellhaving high efiiciency.

Still another object of the invention is to provide an aluminum anodecell having a relatively thin electrolyte of large area.

A further object of the invention is to provide a heat-activated,aluminum anode cell which may be operated at temperatures at which theelectrolyte may be either solid or molten.

An additional object of the invention is to provide an e.m.f. cellhaving an aluminum anode which does not corrode.

It is yet another object of the invention to provide a cell of the abovetype in which the cathode may be either solid or molten.

In summary, the invention provides a heat-activated cell using analuminum anode. The cell operates at high efficiency by using a thin,heated electrolyte comprising at least one aluminum salt. Depending onthe materials used for the cathode and the electrolyte, the cell may beoperated with either the electrolyte or the cathode in a molten state.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view ofone configuration of a cell embodying the invention:

FIG. 2 is a cross-sectional view of a cell in which the anode, cathode,and electrolyte are in the form of side-by-side plates;

FIG. 3 is a pictorial drawing of a rollable cell embodying theinvention; and

FIG. 4 is a cross-sectional view of still another form of cell embodyingthe invention.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to FIG. 1, it willbe seen that a cell constructed in accordance with the invention mayinclude an aluminum container which serves as the anode of the cell. Apost 11 formed of porous carbon saturated with sulfur is disposed in thecontainer 10 to serve as a cathode. The sulfur must be substantiallycompletely and evenly distributed in the p rous carbon of the post 11 inorder for the post to function properly as a cathode. The porous carbonthus serves as a matrix or holder-support for the sulfur cathodematerial. Because methods of saturating porous carbon with sulfur arewell known to those skilled in the art, such methods will not bedescribed herein.

If desired, the cathode post 11 may be formed of any metal which: may bemade in porous form; is lower in the electromotive series than aluminum;and does not react chemically with the sulfur or any other suitablecathode: materials used to saturate the cathode post 11. Other suitablematerials which may replace the sulfur in the post II include any 0 or Cl producing materials, as for example, halogen compounds or organiccompounds containing oxygen. In general, the cathode may be any suitableoxidizing material which will react with aluminum to form an aluminumsalt.

To the end that the cell shown in FIG. 1 will produce a relativelyhigh-current density at high efficiency, a thin electrolyte 12 isdisposed between the cathode post 11 and the aluminum container 10 andis heated by heat applied to the cell. An electrolyte thickness of0neeighth inch is sufficiently thin for the structure shown in FIG. 1.In accordance with the invention, the electrolyte 12 comprises at leastone aluminum salt. At least one alkali halide salt may be added to thealuminum salt to minimize vaporization of the aluminum salt. Because itis generally desirable to operate a heat-activated cell at the lowestpossible temperature, and because of considerations of economy,availability, and ease of handling, a eutectic mixture of AlCl-NaCl hasbeen found to be the most suitable electrolyte. With this particularelectrolyte mixture or with any aluminum halide salt-alkali saltmixture, the cell is normally operated in a temperature range of fromabout 50 C. to about l00 C. by externally applied heat. However, thecell may be operated up to temperatures greater than the melting pointof the electrolyte mixture if desired. The uppermost practical limit isabout 600 C. which is well below the melting point of the aluminumanode.

The cell of FIG. 1 is completed by a cover plate 13 and electrodes l4and 15. The cover plate 13 is an electrical insulating material such asceramic. The electrodes 14 may be either carbon or any suitable metalcompatible with the materials used in the cathode post 11, the anode I0,and the electrolyte I2.

FIG. 2 illustrates an alternate arrangement of the cell shown in FIG. 1and like parts are identified by like numerals. In FIG. 2, the anode 10,the cathode I1, and the electrolyte I2 are in the form of side-by-sideplates. The electrolyte 12 is positioned between the anode I0 and thecathode 11. A suitable container 16' made of electrically nonconductivcmaterial is disposed around the anode 10, the cathode I1, and theelectrolyte 12 to hold them in place.

FIG. 3 pictorially shows another possible arrangement of a cellconstructed in accordance with the invention. Parts in FIG. 3 areidentified by numerals corresponding to like parts in FIGS. 1 and 2. Inthe cell of FIG. 3, the anode I0 is a sheet of aluminum foil. Thealuminum salt-alkali halide electrolyte 12 is coated onto the aluminumfoil as a thin layer. This layer is preferably less than 1 millimeterthick. The cathode lll comprises a sulfur-saturated graphite clothplaced against the electrolyte 12. Suitable metal electrodes 14 and 15of carbon or metal are attached to the cathode I1 and the anode 10,respectively. The cell shown in FIG. 3 advantageously may be eitherfolded or rolled and placed in a hermetically sealed con tainer such asa foil pouch.

A cell constructed as shown in FIG. 3 and having a AlCl-NaCl electrolytewill deliver about 10 milliamperes per square centimeter (ma/cm?) ofelectrolyte area when operated at a temperature slightly below C., themelting point of the electrolyte. The cell delivers about 25 ma./cm.when it operates at about 200 C. The e.m.f. in both cases isapproximately 1.2'volts.

FIG. 4 shows still another form of cell embodying the invention. Partscorresponding to those in FIGS. ll, 2, and 3 are identified by likenumerals. The cell of FIG. 4 includes an anode 10 in the form of analuminum rod which extends into a container 16 through a cover plate 13made of an electrically insulating material. The cathode 11 is amolten-mixture comprising sodium sulfide, sulfur, and water. Anelectrode 17 of electrically conductive material extends through thecover plate 13 into the anode 11. The electrode 17 may be carbon or ametal which will not react with any of the other materials in the cell.

A current path such as a load in the form of a resistor 18 may beconnected between the anode l and the electrode 17 to produce a heatingeffect in the cell. This heating helps sustain operation of the cell. Athin electrolyte 12 of aluminum sulfide is formed on the aluminum anode10. The electrolyte also serves as a separator by preventing the cathode11 material from contacting the anode 10. To allow hydrogen sulfide gasto escape as it accumulates around the electrode 17 during operation ofthe cell, a vent 19 is provided in the cover plate 13.

The cell shown in FIG. 4 is a primary cell and is not rechargeable. Thecell is put into operation in the following manner. Commercial qualitysodium sulfide is placed in the container 16. Heat is then applied tothe container 16 causing the sodium sulfide to melt into its own waterof hydration. This takes place when the sodium sulfide is heated to 50C. or above.' When the sodium sulfide is molten, it comprises sodiumsulfide, sulfur, and water. The alumimum anode l0 and the carbonelecn'ode 17 are then inserted into the molten sodium sulfide, sulfur,and water mixture which form the cathode 11. A protective coating ofaluminum sulfide immediately forms on the aluminum anode 10. Thiscoating serves as both a separator and an electrolyte.

As current flows between the anode l0 and the electrode 17 through theload 18, electrochemical action between the anode 10 and the cathode llprogresses. Aluminum ions build up on the anode 10 while sulfide ionsbuild up on the carbon electrode 17. The sulfide ions accumulating onthe electrode 17 tend to reduce or at least limit the current capabilityor energy density of the cell. However, the sulfide ions will react withthe water in the molten cathode to form H 8. Accordingly, the cell maybe made to provide a relatively highcurrent output by removing the H 8gas which forms at the electrode 17. To this end, the H 8 is allowed toescape through a vent 19 provided in the cover plate 13.

By making the load 18 a relatively low resistance, a relative ly highcurrent will be transferred between the anode 10 and .the electrode 17.This accelerates the electrochemical action taking place between theanode 10, the cathode l1, and the electrolyte l2 and produces heating.Thus, by selecting a load 18 of sufficiently low resistance, enough heatmay be generated in the cell to make its operation self-sustaining.Consequently, it will be seen that, with a properly selected load 18,the exterior heat applied to the cell to initiate operation may now beremoved. Of course, it will be understood by those skilled in the artthat the selection of the load 18 will be affected by otherconsiderations such as the size of the container l6 and the amount ofcathode material 11, aswell as the size of anode l0 and the electrode17.

It will be understood that those skilled in the art, may make changesand modifications to the foregoing heat-activated cell without departingfrom the spirit and scope of the invention as set forth in the claimsappended hereto.

What is claimed is:

1. A method of making an e.m.f. cell comprising the steps of:

disposing sodium sulfide in a container;

heating said container and melting the sodium sulfide in its own waterof hydration and forming a cathode consisting essentially of moltensodium sulfide, sulfur, and water mixture;

placing at least a portion of an aluminum body in said molten sodiumsulfide, sulfur and water mixture and forming a layer of sodium sulfideon said aluminum body, said sodium sulfide serving as an electrolyte andsaid aluminum body serving as an anode; and

placing at least a portion of a body of electrical conductor material insaid sodium sulfide, said electrical conductor body serving as anelectrode.

2. The method of claim 1 including the steps of providing a current pathof predetermined resistance between said anode and said electrodeproducing heating in said cell and then removing the heat applied t2said container.

2. The method of claim 1 including the steps of providing a current pathof predetermined resistance between said anode and said electrodeproducing heating in said cell and then removing the heat applied tosaid container.